Method for Manufacturing a Sheet Metal Component from a Flat Steel Product Provided With a Corrosion Protection Coating

ABSTRACT

A method for manufacturing a sheet metal component including: annealing a flat steel product comprising 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, ≤0.06% P, ≤0.01% S, ≤1.0% Al, ≤0.15% Ti, ≤0.6% Nb, ≤0.01% B, ≤1.0% Cr, ≤1.0% Mo, ≤1.0% Cr+Mo, ≤0.2% Ca, ≤0.1% V, remainder iron and impurities in a continuous furnace under an atmosphere consisting of 0.1-15% hydrogen and remainder nitrogen with a specific dew point and temperature profile; applying a coating consisting of ≤15% Si, ≤5% Fe, in total 0.1-5% of at least one alkaline earth or transition metal and a remainder Al and unavoidable impurities; heating the fat steel product to &gt;Ac3 and ≤1000° C. for a time sufficient to introduce a heat energy quantity&gt;100,000-800,000 kJs; hot-forming the flat steel product to form the component; and cooling at least one section of the component at a cooling rate sufficient to generate hardening structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International ApplicationNo. PCT/EP2020/064836 filed May 28, 2020, and claims priority toInternational Application No. PCT/EP2019/064332 filed Jun. 3, 2019, thedisclosures of which are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for manufacturing a sheet metalcomponent from a flat steel product which is provided with a corrosionprotection coating.

Description of Related Art

Flat steel products are understood here as rolling products, the lengthand width of which are each significantly greater than their thickness.These include steel strips and steel sheets in particular.

Unless explicitly stated otherwise, information on the contents of alloyconstituents is always provided in wt. % in this text.

The proportions of certain components of an atmosphere, in particular anannealing atmosphere, are, on the other hand, indicated in vol. %,unless otherwise noted.

A method of the type indicated at the outset is known from EP 2 993 248A1. A flat steel product is used as the starting product for thismethod, the steel substrate of which consists of so-called “MnB steel”.Steels of this type are standardised in EN 10083-3 and have goodhardenability. They enable reliable process control during hot pressing,through which it is economically possible to still cause martensitehardening in the tool during hot forming without additional cooling. Atypical example of such a steel is the steel known under the designation22MnB5, which can be found in the steel key 2004 under the materialnumber 1.5528. Typically, the market-available fully-settled 22MnB5steel contains, in addition to iron and unavoidable impurities, (in wt.%) 0.10-0.250% C, 1.0-1.4% Mn, 0.35-0.4% Si, up to 0.03% P, up to 0.01%S, up to 0.040% Al, up to 0.15% Ti, up to 0.1% Nb, in total up to 0.5%Cr+Mo, and up to 0.005% B. In order to protect the flat steel productsconsisting of such composite steel against corrosive attacks and at thesame time to minimise the risk of hydrogen absorption during the heatingrequired for hot forming, the flat steel products are provided with analuminium-based corrosion protection coat according to the known method,which contains effective contents of at least one alkaline earth ortransition metal as an additional alloy component of 0.005-0.7 wt. %. Inaddition, Si contents of 3-15 wt. % and Fe contents of up to 5 wt. % mayalso be present in the coat. As the at least one alkaline earth ortransition metal of the protective coat, Mg is preferably used here incontents of 0.1-0.5 wt. %, wherein calcium, strontium, sodium or bariumare also considered alternatively or additionally. The Al-basedprotective coat can be applied to the steel substrate by hot-dipcoating, also known in technical terms as “hot-dip aluminising”, or by agas separation process, e.g. the known PVD (Physical Vapour Deposition)or CVD (Chemical Vapour Deposition).

Special requirements for the manner in which the corrosion protectioncoat is applied to the steel substrate consisting of an MnB steel arenot mentioned in the prior art explained above. In the case of heatingof a board coated in the manner described above in the conventionalmanner under a normal atmosphere over a period of 360-800 s to atemperature of 900° C., due to the presence of the alkaline earth ortransition metal in the coat, at most a minimal hydrogen absorption inthe steel substrate occurs, so that the risk of so-called “hydrogenembrittlement” is minimised.

In practical use, however, it can be seen that, despite this success,components formed from the flat steel products produced in the mannerdescribed above have optimised strengths, but cannot always meet theincreasingly higher requirements that are placed on the behaviour ofsheet metal components manufactured from such flat steel products in thecase of resistance welding and on the adhesion of organic layers, suchas painting and the like, on such sheet metal components.

DE 10 2017 210 201 A1 also deals with a method for manufacturing analuminium-based steel component provided with a metallic anti-corrosivecoat. For this purpose, a flat steel product is provided which consistsof, in wt. %, 0.15 to 0.50% C, 0.50 to 3.0% Mn, 0.10 to 0.50% Si, 0.01to 1.00% Cr, up to 0.20% Ti, up to 0.10% Al, up to 0.10% P, up to 0.1%Nb, up to 0.01% N, up to 0.05% S and up to 0.1% B, the remainder of Feand unavoidable impurities and is coated with an Al coating whichconsists of, in wt. %, 3 to 15% Si, 1 to 3.5% Fe, up to 0.5% alkalineand/or alkaline earth metals, the remainder of Al and unavoidableimpurities. The provided sheet metal is annealed in an oven at atemperature and over a period of time, which are linked to each other bya parameter calculated according to a complex formula. Depending on thefurnace dwell time and the temperature, a so-called interdiffusion zoneis to be formed at the transition between substrate and coating, inwhich no martensitic structure occurs during press hardening, but whichis also not to be assigned to the Al coating. This interdiffusion zoneextends starting from the centre of the flat steel product from thethickness from which there is no more martensitic structure in thecomponent to the thickness from which the iron content of the Al coatingis continuously 85 wt. % and the Al content is continuously 10 wt. %.Information on how the interdiffusion zone could be designed in detailor instructions on how the formation and composition of theinterdiffusion zone could be controlled in a targeted manner with regardto certain surface properties of the coating are also not provided inthis prior art. Instead, the focus here is on considerations forimproving the deformation behaviour of the Al coating, in particular theachievable bending angle.

Against this background, the object has emerged to indicate a methodthat makes it possible to form sheet metal components from a flat steelproduct of the type explained above, which meet the highest requirementsfor their weldability and thereby have optimal conditions for a coatingwith an organic coat, in particular for painting.

SUMMARY OF THE INVENTION

It goes without saying that when carrying out the method according tothe invention, the person skilled in the art not only carries out themethod steps explained herein, but also carries out all other steps andactivities that are usually carried out in the practical implementationof such methods in the prior art if the necessity arises.

In a method according to the invention for manufacturing a sheet metalcomponent from a flat steel product which is provided with a corrosionprotection coating, at least the following work steps are thereforecarried out:

-   a) providing a flat steel product which is produced from a steel    which (in wt. %) consists of 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si,    up to 0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to    0.6% Nb, up to 0.01% B, up to 1.0% Cr, up to 1.0% Mo, wherein the    total of the contents of Cr and Mo is at most 1.0%, up to 0.2% Ca,    up to 0.1% V, and as the remainder of iron and unavoidable    impurities;-   b) annealing the flat steel product in a continuous furnace having    four zones A, B, C, D, which are passed through successively by the    flat steel product and in which the flat steel product is annealed    under an annealing atmosphere consisting in each case of 0.1-15 vol.    % hydrogen and as the remainder of nitrogen as well as technically    unavoidable impurities with a dew point temperature TP_(A), TP_(B),    TP_(C), TP_(D) at an annealing temperature GT_(A), GT_(B), GT_(C),    GT_(D), the following specifications apply:

Zone Dew point temperature TP Annealing temperature GT A −10° C. ≤TP_(A) ≤ −25° C. 800° C. ≤ GT_(A) ≤ 950° C. B −27° C. ≤ TP_(B) ≤ −41° C.800° C. ≤ GT_(B) ≤ 930° C. C −30° C. ≤ TP_(C) ≤ −80° C. 800° C. ≤ GT_(C)≤ 950° C. D −30° C. ≤ TP_(D) ≤ −20° C. 750° C. ≤ GT_(D) ≤ 950° C.

-   c) applying a corrosion protection coating to the flat steel product    obtained in work step b), wherein the corrosion protection coating    consists of (in wt. %) up to 15% Si, up to 5% Fe, in total 0.1-5% of    at least one alkaline earth or transition metal and as the remainder    of Al;-   d) optionally: dress rolling the flat steel product provided with    the corrosion protection coating;-   e) optionally: separating a board from the flat steel product;-   f) heating the flat steel product or the board to a hot forming    temperature which is higher than the Ac3 temperature of the steel of    the flat steel product and does not exceed 1000° C. for a holding    time sufficient to introduce a heat energy quantity Js of more than    100,000 kJs and at most 800,000 kJs into the flat steel product or    the board;-   g) hot forming the flat steel product heated to the hot forming    temperature or the board heated to the hot forming temperature into    the sheet metal component;-   h) cooling at least one section of the component at a cooling rate    sufficient to generate a hardening structure in the section of the    sheet metal component.

The invention is based on the knowledge that for the behaviour of sheetmetal components which are provided with an aluminium-based (“Al-based”)corrosion protection coating, in the case of resistance welding and forthe adhesion of an organic coating, in particular painting, on suchsheet metal components, it is not only the composition of the boundarylayer between the corrosion protection coating and the ambientatmosphere that matters, but in particular that parameters such as theroughness and conductivity of the overall coat also play a decisive rolehere. In this case, the manner of annealing according to the invention(work step b)) prior to applying the corrosion protection coating (workstep c)) creates the conditions for the component processed according tothe invention to have an optimally homogeneous corrosion protectioncoating.

Thus, components produced according to the invention typically have acorrosion protection coating, which is formed by a plurality of layersof different composition. By guiding the dew point and annealingtemperature according to the invention during annealing in thecontinuous annealing furnace to prepare the subsequent application ofthe corrosion protection coating, a significant reduction of the porescontained in the coating is achieved.

Through the annealing parameters selected according to the inventionduring annealing (work step b)) prior to coating, it is achieved thatpure iron (“Fe”) is present on at least 70% of the surface of thefinished annealed flat steel product. This results in a good bonding ofthe subsequently applied Al-based coating, by forming an iron-aluminiumlayer (“Fe—Al layer”) at the transition from the steel substrate to thecorrosion protection coating. On the other hand, iron reaches the layersufficiently and in a homogeneously uniform distribution, through whichthe conductivity of the layer is improved and the behaviour duringresistance welding is thereby optimised.

In the event that the flat steel product provided in work step b) isalready a blank which is directly suitable for forming into thecomponent, work step e) can be omitted. If, on the other hand, the flatsteel product provided is a steel strip or a larger steel sheet, a boardof suitable size is separated from this in work step e).

The flat steel product annealed and coated in the manner according tothe invention (work steps b), c)) or the separated (work step e)) boardare heated to the hot forming temperature (work step f)) for hot forming(work step g)). The iron already present in the homogeneous boundarylayer of the corrosion protection coating can diffuse evenly into thecoating without any significant defects. At the same time, the alkalineearth or transition metal provided according to the invention in thecorrosion protection coating diffuses to the surface due to its oxygenaffinity and forms an oxide layer there. Due to their comparable atomicsize, the iron atoms can exchange places in a 1:1 ratio with thealkaline earth or transition metal atoms and are thus incorporated intothe metal grid, so that at most a negligible number of defects can alsooccur due to the diffusion of the alkaline earth or transition metalatoms. As a result of the reduction of the defects achieved according tothe invention, these defects cannot agglomerate into pores in thecorrosion protection coating of a component according to the invention,so that a component according to the invention is characterised by asignificantly reduced number of pores compared with conventionallyproduced components, for example, produced according to the sample of EP2 086 755 B1.

The effects utilised by the invention occur particularly reliably if theadditionally present alkaline earth or transition metal is magnesium(“Mg”), thus if Mg is present alone or in combination with otherelements belonging to the group of alkaline earth or transition metalsin the contents provided according to the invention in the corrosionprotection coating of a flat steel product processed according to theinvention.

The method according to the invention is suitable for the manufacture ofcomponents from flat steel products with a large thickness spectrum.Thus, flat steel products, whose thickness is 0.6-7 mm, can be processedwith the method according to the invention.

The production of the flat steel products provided in work step a) cantake place in any manner known from the prior art. The method accordingto the invention is in particular suitable for processing flat steelproducts with a thickness of 0.8-4 mm, in particular 0.8-3 mm. Flatsteel products with greater thicknesses of more than 3 mm are typicallyprocessed in the hot-rolled state, while thinner sheets are typicallyprovided in the cold-rolled state.

In work step a) flat steel products can also be provided for the methodaccording to the invention which have obtained different thicknesses bymeans of flexible or partial rolling over length and/or width.Similarly, in work step a) for the method according to the invention,flat steel products composed of different sheet metal blanks weldedtogether or of similarly composed flat steel products and steel stripswhich are welded together and together form the flat steel product to beprocessed can be provided for the process according to the invention.

The flat steel product provided according to the invention in each caseconsists of a steel which has a composition typical for MnB steels. Suchsteels typically have yield strengths of 250-580 MPa and tensilestrengths of 400-720 MPa in the delivered state.

Thus, a flat steel product provided according to the invention consistsof

-   -   0.05-0.5 wt. % of carbon (“C”), wherein the C content is        preferably 0.07-0.4 wt. %,    -   0.5-3 wt. % of manganese (“Mn”), wherein the Mn content is        preferably 0.8-2.5 wt. %, in particular 1.0-2.0 wt. %,    -   0.06-1.7 wt. % of silicon (“Si”), wherein the Si content is        preferably 0.06-1.1 wt. %, in particular 0.06-0.9 wt. %,    -   up to 0.06 of phosphorus (“P”), wherein the P content is at most        0.03 wt. %,    -   up to 0.01 wt. % of sulfur (“S”),    -   up to 1.0 wt. % of aluminium (“Al”), wherein the Al content is        preferably at most 0.5 wt. %, in particular at most 0.1 wt. %,    -   up to 0.15 wt. % of titanium (“Ti”),    -   up to 0.6 wt. % of niobium (“Nb”), wherein the Nb content is        preferably up to 0.1 wt. %,    -   up to 0.01 wt. % of boron (“B”), wherein the B content        preferably up to 0.005 wt. %,    -   up to 1.0 wt. % of chromium (“Cr”), wherein the Cr content is        preferably up to 0.5 wt. %, in particular up to 0.2 wt. %,    -   up to 1.0 wt. % of molybdenum (“Mo”), wherein the Mo content is        preferably up to 0.5 wt. %, in particular up to 0.2 wt. %,    -   wherein for the content % Cr of Cr and the content % Mo of Mo        the following applies % Cr+% Mo 1 wt. %,    -   optionally up to 0.2 wt. %, in particular up to 0.1 wt. %, of        calcium (“Ca”),    -   optionally up to 0.1 wt. % of vanadium (“Va”),        and as the remainder of iron and unavoidable impurities.

Due to their property profile, in particular their potential for thedevelopment of high strengths in the finished hot-formed and cooledcomponent, flat steel products, which in a manner known per se consistof 0.07-0.4 wt. % C, 1.0-2 wt. % Mn, 0.06-0.4 wt. % Si, up to 0.03 wt. %P, up to 0.01 wt. % S, up to 0.1 wt. % Al, up to 0.15 wt. % Ti, up to0.6 wt. % Nb, up to 0.005 wt. % B, up to 0.5 wt. % Cr, up to 0.5 wt. %Mo are particularly interesting in practice, wherein the total of thecontents of Cr and Mo is at most 0.5 wt. %, the remainder consisting ofiron and unavoidable impurities.

This includes steels already in series use, which consist of 0.07-0.4wt. % C, 1.0-1.5 wt. % Mn, 0.3-0.4 wt. % Si, up to 0.03 wt. % P, up to0.01 wt. % S, up to 0.05 wt. % Al, up to 0.15 wt. % Ti, up to 0.6 wt. %Nb, up to 0.005 wt. % B, up to 0.5 wt. % Cr, up to 0.5 wt. % Mo, whereinthe total of the contents of Cr and Mo is at most 0.5 wt. % and consistof iron and unavoidable impurities as the remainder. Such compositesteels achieve tensile strengths of up to 2000 MPa after hot forming andcooling.

As already mentioned, annealing (work step b)) completed in fouruninterrupted successive steps A, B, C, D on the respectively processedflat steel product produces a surface which is largely completelycovered, i.e. to at least 70%, in particular to at least 80% or at least90%, by pure Fe. For this purpose, in zones A-D of the continuousannealing furnace used according to the invention, particularly matcheddew point and annealing temperatures are set in each case.

The annealing carried out in work step b) in zones A-D takes place ineach case under an annealing atmosphere containing 0.1-15 vol. %hydrogen, the remainder of which consists of nitrogen and unavoidableimpurities in each case, wherein the total of the impurities istypically at most 5 vol. %, in particular at most 4 vol. % or preferablyat most 3 vol. %.

All of the information provided below for annealing temperatures GT_(A),GT_(B), GT_(C) and GT_(D) refer to the average furnace chambertemperature during the strip throughput.

Before entering Zone A of the continuous furnace operated according tothe invention, a wide range of oxide products are present on the surfaceof the flat steel product provided according to the invention, whichhave a negative effect with regard to the quality of the coating and inparticular with regard to the pore formation in the coating. Through thecontinuous annealing according to the invention, these oxides areconverted so that, in the technical sense, only Fe is present on thesurface of the flat steel product after the annealing.

By setting the dew point temperature TP_(A) to −10° C. to −25° C. andthe annealing temperature GT_(A) to 800-950° C. in zone A of thecontinuous furnace, the oxides present on the flat steel product areoverlaid with iron oxides. In order to achieve this in a particularlytargeted manner, the annealing temperature GT_(A) can be 810-940° C. andthe dew point temperature TP_(A) can be −15-−25° C. in zone A of thecontinuous furnace.

In zones B and C, the iron oxides are reduced, so that iron is presenton the surface after zone C. In zone B, the dew point temperature TP_(B)of the annealing atmosphere prevailing there is then reduced to −27° to−41° C. and the annealing temperature GT_(B) is maintained at 800-930°C., wherein it has proven to be particularly reliable in terms of thedesired effect if the annealing temperature GT_(B) in zone B of thecontinuous furnace is 800-900° C. in the case of annealing completed inwork step b).

In zone C, the dew point temperature TP_(C) of the annealing atmosphereprevailing there is then further reduced to −30° C. to −80° C. and theannealing temperature GT_(C) is maintained at 800-950° C. in order tocomplete the reduction of the iron oxide into iron. This effect can beachieved particularly reliably if the annealing temperature GT_(C) is800-920° C. and the dew point temperature TP_(C) is −30° C. to −50° C.in the case of the annealing completed in work step b) in zone C of thecontinuous furnace.

In zone D, the dew point temperature TP_(D) of the annealing atmosphereprevailing there is then increased to −30° C. to −20° C. and theannealing temperature GT_(D) is maintained at 750-950° C. in order totemper the flat steel product in such a way that on the one hand itsrecrystallisation can take place and on the other hand the previouslyachieved pure iron surface is retained. This effect can then be achievedparticularly reliably if the annealing temperature GT_(D) is 780-930° C.in the case of the annealing completed in work step b) in zone D of thecontinuous furnace.

The lambda value λ describes the ratio of the masses of air to fuelintroduced into the continuous furnace and in the annealing atmospheremaintained in zones A-D of a continuous furnace used according to theinvention is typically 0.95-1.1 in the case of annealing completed inwork step b) of the method according to the invention.

The prerequisite for the effects achieved according to the invention isthe presence of at least one alkaline earth or transition metal in thealuminium (Al)-based corrosion protection coating applied after theannealing according to the invention (work step b). Thus, in the coat ofa flat steel product processed according to the invention after applyingthe corrosion protection coating (work step c)) and before heating forhot forming (work step f)), at least 0.1-5 wt. % of at least onealkaline earth or transition metal and as the remainder Al andunavoidable impurities are present. Here, alkaline earth or transitionmetal contents of at least 0.11 wt. % have proven to be particularlyfavourable in terms of reliability, with which the positive effects ofthe presence of the at least one alkaline or transition metal in thecoat applied according to the invention can be utilised. If the alkalineearth or transition metal content is over 5 wt. %, increased oxideformation would occur in the melting crucible, which would reduce thesurface quality. In hot forming, too much oxide would also form, whichwould, on the one hand, promote the water fission to hydrogen and oxygenand, as a result, create the risk of more hydrogen entering the steel.On the other hand, the thicker oxide layer can lead to greatercontamination in the forming tool. In order to reliably avoid thiseffect, the content of alkaline earth metal or transition metal in thecorrosion protection coating applied in work step c) can be limited intotal to at most 1.5 wt. %, in particular at most 0.6 wt. %. Thealkaline earth or transition metal contents of the corrosion protectioncoating applied in work step c) are thus in particular 0.11-1.5 wt. %or, in particular, 0.11-0.6 wt. %.

As already mentioned, Mg from the group of alkaline earth or transitionmetals has proven to be particularly suitable for the purposes accordingto the invention, which can be present in the coat applied according tothe invention alone or in combination with other alkaline earth ortransition metals, such as beryllium, calcium, strontium and barium, inorder to be able to use the effects sought according to the invention.

Optionally, silicon (“Si”) can also be present in the coat, which isapplied in work step c) in contents of up to 15 wt. %, in particular upto 11 wt. %, in order to promote the formation of an iron aluminiumlayer, which adheres well to the iron surface set in work step b) andthereby takes up at most one third of the total layer thickness of thecoat. If Si contents are too high, an excessively large alloy layerthickness would result, which in turn could lead to adhesion loss. Sicontents of at least 3 wt. %, in particular at least 8.5 wt. %, prove tobe particularly favourable in this respect, so that with an Si contentof 3-15 wt. %, in particular 3-11 wt. %, in particular 8.5-11 wt. %, thepositive influences of Si can be used particularly reliably in practice.

In addition, in the coat applied in the work step c) Fe can alsooptionally be present in contents of up to 5 wt. %, in particular up to4 wt. %, in particular up to 3.5 wt. %. Iron would be set in this orderof magnitude in the coat because this is the saturation value of analuminium melt in the temperature range 650-720° C. By specificallyadding iron to the melt, the risk of dissolving ferrous components ofthe melting crucible that come into contact with the melt can bereduced. In this respect, Fe contents of at least 1 wt. % prove to beparticularly favourable, so that in practice the positive influences ofFe can be particularly reliably utilised with an Fe content of 1-5 wt.%, in particular 1-4 wt. %, especially 1-3.5 wt. %.

The corrosion protection coating can be applied in work step c) of themethod according to the invention in any known manner. In this case,so-called “hot-dip aluminising” is in particular suitable, in which therespective flat steel product is guided through a suitably heated meltbath composed according to the specifications of the invention. Such ahot-dip coating is in particular suitable for strip-shaped flat steelproducts with a thickness of up to 3 mm. In the case of largerthicknesses, one of the vapour deposition processes (PVD, CVD) mentionedat the outset can also be used to apply the corrosion protectioncoating.

The load of a corrosion protection coating applied according to theinvention in work step c) is typically 30-100 g/m², in particular 40-80g/m² per side. The load on both sides of the coating is thus 60-200 g/m²in total.

After applying the corrosion protection coating (work step c)), thecorrespondingly coated flat steel product can optionally be subjected todress rolling (work step d)) in order to set the mechanicalcharacteristic values of the flat steel product, to adjust its surfaceroughness or to homogenise it. The forming degrees set for this (formingdegree=(thickness before dress rolling−thickness after dressrolling)/(thickness before dress rolling)) are typically 0.1-5%.

After applying the corrosion protection coating (work step c)) or theoptionally performed dress rolling (work step d)) a board is, ifrequired, separated from the flat steel product in a manner known perse, the dimensions of which are adapted in a known manner to thedimensions of the sheet metal component to be hot-formed from it (workstep e)).

The flat steel product itself or the board is then heated in work stepf) to a hot forming temperature which is higher than the Ac3 temperatureof the steel of the flat steel product and does not exceed 1000° C., inparticular is at least equal to the Ac3 temperature+50° C. and is atmost 980° C., wherein hot forming temperatures of 820-950° C. haveproven to be particularly advantageous. The flat steel product is heldat this temperature until a sufficient amount of heat is introduced intothe flat steel product or the board separated therefrom. The holdingtime and annealing temperature required in each case can be estimated onthe basis of the proviso that the heat energy quantity Js introducedinto the flat steel product or the board in work step f) should be morethan 100,000 kJs and at most 800,000 kJs, wherein Js can be calculatedaccording to the following known equation:

Js[kJs] = [(T 2 − T 1) × c × t × m]/1000;

with

-   -   T2: End temperature of the component at the end of heating in K    -   T1: Start temperature of the component at the start of heating        in K    -   c: Heat capacity steel (typically 460 J/kgK)    -   t: Holding time of the flat steel product or the board at the        end temperature in s    -   m: Mass of the flat steel product or the board in kg

Heating can be carried out in any suitable way. In the event that aconventional continuous furnace is used for this purpose, in which theflat steel product or the board is heated by radiant heat, the suitableholding time is typically 100-900 s, preferably 180-720 s, in particular240-600 s. In the event that a hot forming temperature of 850-930° C. isselected, holding times of 180-600 s, in particular 240-600 s, aregenerally sufficient in practice. As an alternative to the use of acontinuous furnace, it is also possible, for example, to carry outheating in a conventional chamber furnace.

The heating of the flat steel product or the board can also take placein two steps in a manner also known per se in order to initially achievea pre-alloying of the corrosion protection coating and subsequentlybring the flat steel product or the board to the respective hot formingtemperature.

The board heated to the hot forming temperature or the flat steelproduct heated to the hot forming temperature is inserted into the hotforming tool within a transfer time of typically less than 15 seconds,in particular less than 10 seconds, and then hot-formed there into thecomponent (work step g)).

Subsequently or simultaneously, at least one section of the componentobtained is cooled in a manner that is controlled and known per se, inorder to generate the desired structure in the relevant section of thecomponent. The cooling rates required for this are typically 20-500 K/s,wherein cooling rates of more than 30 K/s, in particular more than 50K/s, are particularly practical. Cooling “of at least one section” alsoof course includes the possibility of cooling the component as a wholein the aforementioned manner in order to generate hardening structuresin the entire component.

With the method according to the invention, the production of a sheetmetal component, which is manufactured from a flat steel product, thesteel substrate of which consists of a steel, which (in wt. %) consistsof 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, up to 0.06% P, up to 0.01% S,up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0%Cr, up to 1.0% Mo, wherein the total of the contents of Cr and Mo is atmost 1.0%, up to 0.2% Ca, in particular up to 0.1% V, and as theremainder of iron and unavoidable impurities, and which is coated with acorrosion protection coating consisting of (in wt. %) up to 15% Si, upto 5% Fe, in total 0.1-5% of at least one alkaline earth or transitionmetal and as the remainder of Al and unavoidable impurities, wherein thelayer of the corrosion protection coating adjoining the steel substrateis an interdiffusion layer consisting of ferrite with an Al content ofup to 50 wt. %, in particular at least 1 wt. % Al, wherein in across-section of the interdiffusion layer, the proportion of the surfacecovered by pores with a diameter 0.1 μm is less than 10%, in particularless than 5%, preferably less than 3%, and wherein the surface coveredwith pores in the interdiffusion layer is <300 μm², in particular lessthan 200 μm², particularly preferably less than 100 μm² over ameasurement length of 500 μm. The thickness of the alloy layer here is1-30 μm, preferably 2-20 μm, in particular 4-16 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a steel sheet of a sheet metal componentmanufactured according to the invention by hot forming in 500×magnification. The cross-section was prepared in a conventional mannerby etching with 3% Nital in order to clarify the layer structure presenton the steel sheet.

FIG. 2 shows a schematic representation of the cross-section accordingto FIG. 1.

DESCRIPTION OF THE INVENTION

The corrosion protection coating K formed on the steel substrate Scomprises an interdiffusion layer D directly connected to the steelsubstrate S, which substantially consists of alpha mixed crystal (i.e.ferrite) with increased Al content. Fe2Al5 is still present here inphases. The interdiffusion layer D is characterised in that it ishomogeneously and uniformly formed and that it is virtually pore-free.

In the direction of the free surface O of the corrosion protectioncoating K, a first Si-rich layer S₁ has formed on the diffusion layer D.At the boundary between the diffusion layer D and the Si-rich layer S₁,pores P1 are present in the diffusion layer D in small numbers and farapart from one another.

In the direction of the free surface O on the Si-rich layer S₁, a firstintermediate layer Z₁ has formed, which consists of aluminium iron,wherein the majority lies in the aluminium. Traces of Si, alkaline earthand/or transition metals as well as unavoidable impurities may also bepresent in the layer Si. The intermediate layer Z1 is pore-free.

In the direction of the free surface O on the intermediate layer Z₁there is a second Si-rich layer S₂.

A second intermediate layer Z₂ is formed in the direction of the freesurface O on the Si-rich layer S₂. The layer Z2 also consists ofaluminium iron, with the majority being aluminium and alkaline earthand/or transition metals may also be present. Traces of Si as well asunavoidable impurities may also be present. The intermediate layer Z2 isalso pore-free.

The second intermediate layer Z2 is covered on its side facing the freesurface O with an oxide layer OX, which substantially consists ofaluminium, silicon and alkaline earth and/or transition metal oxides.Oxide layer thicknesses of up to 1.5 μm can be present on average on ahot-formed component. Crater-shaped pores P₂, which are open to theenvironment, have formed in a small number and at large distance fromone another on the surface of the oxide layer OX forming the freesurface O of the corrosion protection coating K.

For comparison, a component was formed from a flat steel product whichwas covered with an AlSi coating according to the sample of the priorart described in EP 2 086 755. Its coating consisted of (in wt. %) 9.5%Si, 3.5% Fe and, as the remainder of aluminium and unavoidableimpurities, was therefore free of alkaline earth or transition metals ofthe type added according to the invention.

The steel substrate of the flat steel product consisted of (in wt. %)0.224% C, 0.25% Si, 1.16% Mn, 0.014% P, 0.002% S, 0.039% Al, 0.0034% N,0.2% Cr, 0.03% Ti and 0.0026% B.

Before applying the metallic coating and forming into the flat steelproduct, the flat steel product processed for comparison has undergonean annealing treatment in a continuous furnace with four zones in whichthe dew point temperatures TP and annealing temperatures GT indicated inTable 6 have been set. The air ratio A in the continuous furnace was0.98.

A five-layered layer structure of the corrosion protection coating hasalso been created for the component produced conventionally forcomparison. However, compared to the number of pores in the coat of thecomponent produced conventionally for comparison, in the componentproduced according to the invention, the number of pores P2 in the oxidelayer OX was reduced by at least 25% and the number of pores P1 in thediffusion layer D by at least 40% compared with the pores present in thecorresponding layers of the corrosion protection coating of thecomponent produced conventionally for comparison. The area covered withpores P1 was 300 μm² after a dwell time in the furnace of 600 s with ameasurement length of 500 μm in the layer D.

The reduction of pores in P2 leads to a reduction of paint craters andimproves adhesion and weldability. The pores in P2 have openings in thedirection of the atmosphere of a few nm. If a component is now processedfurther after hot forming as is typical for cars, it will undergocathodic dip painting in addition to a larger number of cleaning steps.Contact with water-based solutions is unavoidable here. During cleaning,water can penetrate into the pores P2 of the layer, since thesurfactants added to the cleaning water improve wetting andsignificantly reduce the surface tension of the water. Water can alsopenetrate the opened pores P2 in the cathodic dip painting process. Inthis particular case, the cleaning water also leads to a separation ofthe paint particles, which cannot penetrate into the pores P2 due to thesize of the opening. Water, which is then present in the pores P2,reaches the boiling point when the paint layers are baked in, whichleads to vapour phases which, in a kind of boiling delay, escapesexplosively through the paint to the environment. As a result of thisreaction, so-called paint craters form, which, in addition to visualinfluence, also significantly reduce the effect of the paint in terms ofcorrosion protection. In the case of aluminium-based coats inparticular, corrosion and paint infiltration can occur at such points.The red rust that occurs, which is formed due to the high iron contentsof the coating and stands out visually, is particularly problematic forthe further processor.

Also, on a surface where many open pores P2 are present, adhesivescannot penetrate into the pores P2 due to their higher viscosity. Thismay result in incomplete coverage of the surface with adhesive. Cavitiesalso form in the area of the pores, as a result of which adhesion isalso impaired.

The pores P2 present in the layer OX also lead to changed current pathsin the material during resistance spot welding, which negativelyinfluence the weldability.

In the case of a high pore count, there is also an enlarged surface onwhich water can split during oxidation in the hot forming process. Inthis way, diffuse hydrogen can penetrate into the material, which isknown to increase the risk of hydrogen-induced cracking.

By minimising the frequency at which the pores P2 occur during themanufacture of a sheet metal component according to the invention, therisks associated with pore formation in conventionally producedcomponents can be effectively reduced.

The reduction of the number of pores P1 in diffusion layer D also leadsto an increase in the transferable force of adhesive bonds and to animprovement in the weldability.

The pores in P2 represent cavities within the corrosion protectioncoating K. If the number of pores is too high, there is a risk that thecorrosion protection coating K will break up at the boundary regionbetween the diffusion layer D and the first Si-rich layer S1, with theresult that the adhesive seam also fails at an early stage. With thereduction of the number of pores P1 achieved according to the invention,the area over which the forces of the adhesive bond are transferred isincreased by over 60% and thus the risk of delamination fracture iscorrespondingly reduced.

In order to prove the effect of the invention, steel sheets each with athickness of 1.5 mm and cold-rolled in a conventional manner have beenproduced from six steels ST1-ST6, the compositions of which areindicated in Table 1 (work step a) of the method according to theinvention).

The steel sheets provided in this way were subjected in nine tests V1-V9in each case to a continuous annealing G1, G2 or G3 in a continuousfurnace, which had four consecutive zones A, B, C, D. Table 2 shows thedew point temperatures TP_(A)-TP_(D) set in zones A-D for variants G1-G3of the annealing, the annealing temperatures GT_(A)-GT_(D) as well asthe hydrogen content H2 and the nitrogen content N2 of the respectiveannealing atmosphere, the remainder of which consisted of technicallyunavoidable impurities (work step b) the method according to theinvention).

The samples annealed in this way are each coated in a conventionalmanner with a Al-based corrosion protection coating Z1-Z5 with a loadAG. The compositions of the corrosion protection coatings Z1-Z5 areindicated in Table 3 (work step c) of the method according to theinvention).

The samples each provided with one of the corrosion protection coatingsZ1-Z5 were heated in each case in the tests V1-V9 in the continuousfurnace to a hot forming temperature T_(WU) at which they were held fora holding time t_(WU) (work step f) of the method according to theinvention.

The steel ST1-ST6, of which the samples each used in the tests V1-V9consisted, the variants G1-G3 of the annealing each used in the testsV1-V9, the compositions Z1-Z5 of the corrosion protection coatings eachproduced in the tests V1-V9 and their respective loads AG as well as thehot forming temperatures T_(WU) and holding times t_(WU) each selectedin the tests V1-V9 are indicated in Table 4.

The samples heated in this way were taken from the continuous furnace ina transfer time of 3-7 s in each case and placed in a conventional hotforming tool in which they were hot-formed into a component.Subsequently, cooling took place at 270 K/s in each case to roomtemperature (work steps g) and h) the method according to the invention.

Of the components obtained in the tests V1-V9, three cross-sections wereproduced in a manner known per se, which were etched with 3% Nital toclarify the layer structure. Illustrations of the cross-sections weregenerated in 500× magnification, as shown by way of example in FIG. 1.In the respective illustration, the pores P1, P2 present in the layersOX and D were counted over a section with a length of 550 μm. Thearithmetic mean was formed from the counter results determined for thethree cross-sections of a sample in each case. This arithmetic mean ofthe numbers determined for the pores P1 and P2 has been compared withthe comparative values determined in the same way for a comparativesample.

The relative reduction in pore counts P1 and P2 resulting from thiscomparison and achieved by the invention is indicated in Table 5. Table5 also shows the proportion of paint craters in the total area of therespective sample, the decrease in the delamination area and the weldingregion determined in accordance with the steel-iron test sheet SEP1220-2. Welding regions greater than 1 kA have been classified as “OK”.

TABLE 1 Steel C Si Mn P S Al Nb Ti B A 0.08 0.33 0.95 0.025 0.02 0.0130.09 0.01 0.005 B 0.23 0.38 1.3 0.02 0.007 0.013 — 0.03 0.004 C 0.380.37 1.38 0.02 0.008 0.013 — 0.1 0.005 D 0.2 0.35 1.35 0.02 0.008 0.012— 0.02 0.004 E 0.14 0.25 1.07 0.1 0.001 0.08 0.025 0.01 0.002 F 0.24 0.31.3 0.022 0.008 0.012 — 0.02 0.004 Information in wt. %, the remainderFe and unavoidable impurities

TABLE 2 Annealing Dew point Annealing atmosphere temperature TPtemperature GT GA Lambda [° C.] [° C.] [vol %] Annealing value A B C D AB C D H2 N2 G1 1.05 −25 −40 −40 −20 880 880 850 800 7 91 G2 1.1 −20 −40−45 −25 890 890 830 800 10 87 G3 0.95 −12 −30 −47 −22 890 900 900 820 592

TABLE 3 Corrosion protection coating Mg Si Fe Z1 0.3 9.5 3 Z2 0.5 8 3.5Z3 0.1 10 3 Z4 2 8 2 Z5 0.8 8 3 Information in wt. %, remainder Al andunavoidable impurities

TABLE 4 Corrosion protection AG Twu twu Test Steel Annealing coating[g/m²] [° C.] [s] V1 A G1 Z3 69 920 300 V2 B G2 Z2 70 920 180 V3 C G1 Z375 925 360 V4 D G3 Z5 65 920 420 V5 E G1 Z1 70 900 300 V6 F G3 Z4 71 920360 V7 H G2 Z1 65 925 360 V8 B G1 Z3 72 920 300 V9 D G3 Z2 71 925 300

TABLE 5 Reduction Reduction Decrease in of of Paint delamination Weldingpores P1 pores P2 craters fracture area region Test [%] [kA] V1 25 508.4 67 1.1 V2 30 75 10.5 70 1 V3 28 43 9.5 62 1.2 V4 35 52 10.9 60 1.1V5 33 60 11.6 65 1.2 V6 25 58 7.8 63 1 V7 37 57 12.5 60 1.1 V8 33 7011.6 70 1 V9 28 65 9.5 69 1.2

TABLE 6 T_(P) T_(G) Zone [° C.] [° C.] A −30 750 B −23 780 C −25 780 D−35 740

1. A method for manufacturing a sheet metal component from a flat steelproduct provided with a corrosion protection coating, the methodcomprising: a) providing a flat steel product produced from a steelwhich (in wt. %) comprises 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, up to0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, upto 0.01% B, up to 1.0% Cr, up to 1.0% Mo, wherein the total of thecontents of Cr and Mo is at most 1.0%, up to 0.2% Ca, up to 0.1% V, andas the remainder of iron and unavoidable impurities; b) annealing theflat steel product in a continuous furnace having four zones A, B, C, D,which are passed through successively by the flat steel product and inwhich the flat steel product is annealed under an annealing atmosphereconsisting in each case of 0.1-15 vol. % hydrogen and as the remainderof nitrogen as well as technically unavoidable impurities with a dewpoint temperature TP_(A), TP_(B), TP_(C), TP_(D) at an annealingtemperature GT_(A), GT_(B), GT_(C), GT_(D), the following specificationsapply: one Dew point temperature TP Annealing temperature GT −10° C. ≤TP_(A) ≤ −25° C. 800° C. ≤ GT_(A) ≤ 950° C. −27° C. ≤ TP_(B) ≤ −41° C.800° C. ≤ GT_(B) ≤ 930° C. −30° C. ≤ TP_(C) ≤ −80° C. 800° C. ≤ GT_(C) ≤950° C. −30° C. ≤ TP_(D) ≤ −20° C. 750° C. ≤ GT_(D) ≤ 950° C.

c) applying a corrosion protection coating to the flat steel productobtained in work step b), wherein the corrosion protection coatingconsists of (in wt. %) up to 15% Si, up to 5% Fe, in total 0.1-5% of atleast one alkaline earth or transition metal and as the remainder of Aland unavoidable impurities; d) optionally: dress rolling the flat steelproduct provided with the corrosion protection coating; e) optionally:separating a board from the flat steel product; f) heating the flatsteel product or the board to a hot forming temperature which is higherthan the Ac3 temperature of the steel of the flat steel product and doesnot exceed 1000° C. for a holding time sufficient to introduce a heatenergy quantity Js of more than 100,000 kJs and at most 800,000 kJs intothe flat steel product or the board; g) hot forming the flat steelproduct heated to the hot forming temperature or the board heated to thehot forming temperature into the sheet metal component; h) cooling atleast one section of the component at a cooling rate sufficient togenerate a hardening structure in the at least one section of the sheetmetal component.
 2. The method according to claim 1, wherein a thicknessof the flat steel product provided in work step a) is 0.6-7 mm.
 3. Themethod according to claim 1, wherein the annealing temperature GT_(A) is810-940° C. and the dew point temperature TP_(A) is −15° C. to −25° C.in zone A of the continuous furnace in the annealing completed in workstep b).
 4. The method according to claim 1, wherein the annealingtemperature GT_(B) is 800-900° C. in zone B of the continuous furnace inthe annealing completed in work step b).
 5. The method according toclaim 1, wherein the annealing temperature GT_(C) is 800-920° C. and thedew point temperature TP_(C) is −30° C. to −50° C. in zone C of thecontinuous furnace in the annealing completed in work step b).
 6. Themethod according to claim 1, wherein the annealing temperature GT_(D) is780-930° C. in zone D of the continuous furnace in the annealingcompleted in work step b).
 7. The method according to claim 1, wherein alambda value λ of the annealing atmosphere maintained in zones A-D is0.95-1.1 in the annealing completed in work step b).
 8. The methodaccording to claim 1, wherein the Si content of the corrosion protectioncoating applied to the flat steel product in work step c) is at least 3wt. %.
 9. The method according to claim 1, wherein the Fe content of thecorrosion protection coating applied to the flat steel product in workstep c) is at least 1 wt. %.
 10. The method according to claim 1,wherein the corrosion protection coating applied to the flat steelproduct in work step c) contains in total at least 0.11 wt. % ofalkaline earth or transition metals.
 11. The method according to claim1, wherein the content of alkaline earth or transition metals in thecorrosion protection coating applied to the flat steel product in workstep c) is in total at most 0.6 wt. %.
 12. The method according to claim1, wherein the corrosion protection coating applied to the flat steelproduct in work step c) contains magnesium as the at least one alkalineearth or transition metal.
 13. The method according to claim 1, whereinan amount of the corrosion protection coating applied to the flat steelproduct in work step c) is 30-100 g/m² per coated side of the flat steelproduct.
 14. The method according to claim 1, wherein the application ofthe corrosion protection coating in work step c) takes place by hot-dipcoating.
 15. The method according to claim 1, wherein the heating of theflat steel product or the board in work step f) takes place in acontinuous furnace by radiant heat and the holding time is 100-900 s.16. A sheet metal component manufactured from a flat steel product, thesteel substrate of which consists of a steel, which (in wt. %) comprises0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, up to 0.06% P, up to 0.01% S, upto 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0% Cr,up to 1.0% Mo, wherein the total of the contents of Cr and Mo is at most1.0%, up to 0.2% Ca, up to 0.1% V, and as the remainder of iron andunavoidable impurities, and which is coated with a corrosion protectionconsisting of (in wt. %) up to 15% Si, up to 5% Fe, in total 0.1-5 wt. %of at least one alkaline earth or transition metal and as the remainderof Al and unavoidable impurities, wherein the layer of the corrosionprotection coating adjoining the steel substrate is an interdiffusionlayer (D) consisting of ferrite with an Al content of up to 50 wt. %,wherein in a cross-section of the interdiffusion layer (D), theproportion of the surface covered by pores with a diameter ≥0.1 μm isless than 10% and wherein the surface covered with pores in theinterdiffusion layer (D) is <300 μm² over a measurement length of 500μm.
 17. The sheet metal component according to claim 16, wherein theinterdiffusion layer (D) has a thickness of 1-30 μm.